Method and apparatus for improved ion bunching in an ion implantation system

ABSTRACT

An ion buncher stage for a linear accelerator system is disclosed for bunching ions in an ion implantation system. The ion buncher stage may be employed upstream of one or more accelerating stages such that the loss of ions in the linear accelerator system is reduced. The invention further includes an asymmetrical double gap buncher stage, as well as a slit buncher stage for further improvement of ion implantation efficiency. Also disclosed are methods for accelerating ions in an ion implanter linear accelerator.

RELATED APPLICATION

[0001] This application claims priority to Ser. No. 60/314,392 filed Aug. 23, 2001, which is entitled “Method and Apparatus for Improved Ion Bunching in an Ion Implantation System”, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to ion implantation systems, and more specifically to methods and apparatus for improved ion bunching in an ion implantation system.

BACKGROUND OF THE INVENTION

[0003] In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. A high energy (HE) ion implanter is described in U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, which is hereby incorporated by reference as if fully set forth herein. Such HE ion implanters are used for deep implants into a substrate in creating, for example, retrograde wells. Implant energies of 1.5 MeV (million electron volts), are typical for such deep implants. Although lower energy may be used, such implanters typically perform implants at energies between at least 300 keV and 700 keV. Some HE ion implanters are capable of providing ion beams at energy levels up to 5 MeV.

[0004] Referring to FIG. 1, a typical high energy ion implanter 10 is illustrated, having a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes an ion source 20 powered by a high voltage power supply 22. The ion source 20 produces an ion beam 24 that is provided to the beamline assembly 14. The ion beam 24 is then directed toward a target wafer 30 in the end station 16. The ion beam 24 is conditioned by the beamline assembly 14 that comprises a mass analysis magnet 26 and a radio frequency (RF) linear accelerator (linac) 28. The linac 28 includes a series of accelerating modules 28 a-28 n, each of which further accelerates ions beyond the energies they achieve from prior modules. The accelerating modules maybe individually energized by a high RF voltage that is typically generated by a resonance method to keep the required average power reasonable. The mass analysis magnet 26 passes only ions of an appropriate charge-to-mass ratio to the linac 28.

[0005] The linear accelerating modules 28 a-28 n in the high energy ion implanter 10 individually include an RF amplifier, a resonator, and an energizable electrode. The resonators, for example, as described in U.S. Pat. No. 4,667,111 operate at a frequency in the range of about 3-30 Mhz, with a voltage of about 0 to 150 kV, in order to accelerate ions of the beam 24 to energies over one million electron volts per charge state. As the ion beam 24 travels through the various accelerating modules or stages 28, some of the ions therein are properly accelerated, whereas others are not. Thus, conventional linear accelerators 28 achieve an ion acceleration efficiency that is less than 100%. In particular, conventional ion accelerators may deliver less than 20% of the ions from the mass analysis magnet 26 to the target,wafer 30 in the end station 16. In particular, each ion accelerating stage 28 a-28 n is tuned or adjusted in order to provide appropriate acceleration to ions provided thereto which fall within a tolerance or acceptance range. Maintaining ions in an ion beam is facilitated where the ions are bunched or packetized prior to acceleration, whereby a larger percentage of ions are accelerated by each accelerating module or stage 28. In conventional linear accelerators 28, the first or the first and second accelerating modules (e.g., modules 28 a, 28 b) may operate as a combination buncher and accelerator. However, this bunching provides limited ion transfer efficiency. Thus, typical linear accelerators 28 may achieve less than 20% transmission of ions. In ion implantation devices, it is desirable to impart ions onto a workpiece, such as a semiconductor product, in a controlled fashion. In conventional systems where approximately 80% of ions generated by an ion source therein may be lost (e.g., not provided to the workpiece), more time is needed to perform the desired implantation. Thus, there is a need for improved methods and apparatus for bunching ions in an ion implantation linear accelerator, in order to increase the percentage of generated ions that are imparted onto a workpiece.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to a linear accelerator having an ion buncher associated therewith that achieves improved ion transport in an ion implantation system. The invention provides a dedicated buncher stage for ensuring that a greater percentage of the generated ions are provided in the region of acceleration for the accelerating stage of the ion implanter, than was heretofore possible. The buncher stage may be positioned upstream of a linear accelerator stage along a beam path in an ion implantation system in order to provide bunches or packets of ions to the accelerating stage of the accelerator. In particular, the invention provides for beam transmission of up to 60%, for example, of the available ions through the linear accelerator of an ion implantation system. Thus, the invention provides significant advantages over conventional ion implantation devices and methodologies in which in some cases less than 20% of available ions were properly accelerated.

[0007] One aspect of the invention provides an asymmetrical double gap buncher providing further advantages and efficiencies associated with ion transfer in an ion implantation system. First and second gaps are provided before and after a buncher modulating electrode, respectively, wherein the gaps differ in size, for example, wherein the second gap is larger than the first gap.

[0008] The modulating field in the buncher accelerates certain ions with respect to a reference ion, and decelerates others with respect thereto. In the drift region, the accelerated ions catch up to the reference ion and the decelerated ions slow down to allow the reference ion (e.g., as well as the accelerated ions) to catch up, thereby providing a net bunching effect. The asymmetry of the gaps facilitates the provision of a higher percentage of available ions to subsequent linear accelerator stages, thereby significantly improving the ion transfer efficiency of ion implantation systems. The asymmetrical double gap buncher may be included as a buncher stage within a linear accelerator system for an ion implanter in which the modulating field strength created by the buncher electrode may be significantly lower than that of the accelerating alternating electric field of the linear accelerator. In this manner, the buncher electrode operates to modulate the DC ion beam (e.g., obtained from an upstream mass analysis magnet), and the drift region allows bunching to occur as a result of the modulation.

[0009] In accordance with a further aspect of the invention, there is provided a slit double gap buncher stage and a modulating electrode therefor, which provide further advantages associated with ion implantation. The modulating electrode comprises an elongated slit aperture in an electrode base extending longitudinally through the base along the ion beam path. The slit may comprise an aspect ratio greater than one, for example, in which the slit height is greater than the slit width. The slit aperture allows reduced gap lengths in the buncher compared to circular electrode apertures, resulting in efficient modulation over a wider range of ion velocities. In addition, the slit double gap buncher may be located between matching quadrupole focusing devices and an entrance aperture in a linear accelerator. In this case, the matching quadrupoles may serve to form the bunched ion beam into a circular profile for injection into the first accelerating stage, as well as to provide a buncher drift region. This provides for reduction in length for an ion implantation device.

[0010] To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is schematic a block diagram illustrating a high energy ion implanter having a linear accelerator;

[0012]FIG. 2 is a schematic diagram illustrating a conventional linear accelerator module;

[0013]FIG. 3 is a perspective view of a portion of a linear ion accelerator which may be employed in an ion implantation system;

[0014]FIG. 4 is a schematic illustration of an exemplary ion implantation system having a linear accelerator and a buncher in accordance with an aspect of the invention;

[0015]FIG. 5 is a schematic illustration of an exemplary buncher and linear accelerator according to the invention;

[0016]FIG. 6 is a perspective view of an exemplary asymmetrical double gap buncher in accordance with another aspect of the invention;

[0017]FIG. 7 is a schematic illustration of a portion of another exemplary linear accelerator in accordance with the invention;

[0018]FIG. 8 is a perspective view of an exemplary slit double gap buncher in accordance with another aspect of the invention; and

[0019]FIG. 9 is a schematic illustration of a portion of another exemplary linear accelerator in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will now be described with reference to the drawings wherein like reference numerals are used to refer to like elements throughout. A dedicated ion buncher stage is provided for bunching ions in an ion implantation system. The ion buncher stage may be employed in a linear accelerator upstream of an accelerating stage to provide ions thereto in bunches or packets, such that the loss of ions in the linear accelerator is reduced. The buncher stage may operate at lower energy levels than the subsequent accelerator stages, and may further provide a drift region to facilitate ion bunching. The invention further includes an asymmetrical double gap buncher stage, as well as a slit buncher stage for further improvement of ion implantation efficiency. Also described hereinafter are methods for accelerating ions in an ion implanter linear accelerator.

[0021] In order to provide context for the features of the invention, a brief discussion of a conventional interconnection for an RF amplifier and resonator in a linear accelerator module (e.g., modules 28 a-28 n of FIG. 1) is now provided. Referring now to FIG. 2, a conventional resonator circuit 100 is illustrated which includes an inductor coil L connected in parallel with a resistance R_(L) and a capacitance C_(S). An energizable electrode 108 is connected to the inductor L. The electrode 108 is mounted between two grounded electrodes 112 and 114, and the energizable electrode 108 and the grounded electrodes 112 and 114 operate to accelerate the ion beam 110. The capacitance C_(S) represents the capacitance of the energizable electrode 108, and the resistance R_(L) represents the losses associated with the resonant circuit comprising the inductor L and the capacitance C_(S). The values for the capacitance C_(S) and the inductor coil L are selected to form a low loss (high Q) resonant or “tank” circuit 100, wherein each accelerator module in a linear accelerator system of the type shown in FIG. 1 resonates at the same frequency. A radio frequency (RF) signal is connected from a matching network (not shown) at point 116 and is capacitively coupled to a high voltage end of the coil L via a capacitor C_(C).

[0022] A perspective view of a portion of a modular linear accelerator 228 is provided in FIG. 3. A DC ion beam 224 a is provided to the accelerator 228 (e.g., from an upstream mass analyzer magnet, not shown), along a beam path 226. The DC beam 224 a may comprise, for example, an elongated slit profile that is passed through an entrance aperture 230 having a vertically elongated slit 232 along the path 226. The beam 224 a is formed into a generally circular profile (not shown) via two sets of matching quadrupole devices 234 and corresponding grounded electrodes 236, wherein the grounded electrodes 236 each comprise a cylindrical aperture 238 located along the path 226.

[0023] The linear accelerator 228 in the present example further comprises two or more accelerating modules or stages 228 a, 228 b, . . . 228 n, where n is an integer, two of which (e.g., stages 228 a and 228 b) are illustrated in FIG. 3. Each of the accelerator modules 228 n further accelerates ions from the beam 224 beyond the energies they achieve from prior modules. The accelerating modules 228 n may be individually energized by high RF voltages generated by a power supply and resonator (not shown). As the ion beam 224 travels through the various accelerating modules or stages 228 n, some of the ions therein are accelerated, whereas others are not. The accelerating module 228 a comprises a pair of grounded electrodes 246 located before and after an energizable electrode 248 along the path 226, where the energizable electrode 248 may be energized by an appropriate RF energy source and resonator (not shown) in order to achieve acceleration of ions within the beam 224 a along the beam path 226. The grounded electrodes 246 are generally equally spaced from the energizable electrode 248 to provide first and second generally equal gaps 250 a and 250 b therebetween. Similarly, the second accelerator module or stage 228 b comprises a first grounded electrode 256 located along the path 226 upstream of a second energizable electrode 258.

[0024] A quadrupole device 264 (for example, an electrostatic quadrupole) may be provided along the path 226 between the first and second accelerator stages 228 a and 228 b to provide for radial focusing of the beam 224 as it travels through successive accelerator stages 228. The accelerator 228 may comprise further accelerating stages or modules (not shown), whereby an ion beam 224 b may be accelerated to an energy higher than that of the DC beam 224 a provided to the accelerator 228.

[0025] Referring now to FIG. 4, an aspect of the invention provides an ion implanter 310 comprising a terminal 312, a beamline assembly 314, and an end station 316. The terminal 312 includes an ion source 320 powered by a high voltage power supply 322. The ion source 320 produces an ion beam 324 that is provided to the beamline assembly 314. The ion beam 324 is then directed toward a target wafer 330 in the end station 316. The ion beam 324 is conditioned by the beamline assembly 314 that comprises a mass analysis magnet 326 and a radio frequency (RF) linear accelerator (linac) 328. The linac 328 includes a buncher stage 340 and a series of accelerating stages or modules 328 a-328 n, each of which further accelerates ions beyond the energies they achieve from prior modules. The accelerating stages or modules may be individually energized by a high RF voltage that is typically generated by a resonance method to keep the required average power reasonable. The accelerating stages or modules 328 a-328 n in the high energy ion implanter 310, for example, individually include an RF amplifier, a resonator, and an accelerating electrode (not shown).

[0026] In accordance with one aspect of the invention, the ion buncher stage 340 is located at the front of the linear accelerator between the mass analysis magnet 326 and the first accelerating stage 328 a. As illustrated and described further hereinafter, the ion buncher stage 340 may comprise a buncher electrode located along the path 324, which is operatively associated with a buncher energy source (not shown) to create a modulating alternating electric field. This modulating field operates on the DC ion beam provided by the mass analysis magnet 326 in order to provide bunched ions to the accelerating stage 328 a. The implanter 310 may further comprise controls (not shown), whereby the terminal 312, beamline assembly 314, and end station 316 may be operated to affect ion implantation in the workpiece 310 in a controlled fashion. In this regard, control of the modulating field in the buncher stage 340 as well as the accelerating fields in the linac 328 may be implemented through such controls.

[0027] Referring briefly to FIG. 5, a schematic illustration is provided of an exemplary buncher stage 350 operating on a DC ion beam 352 in order to group or “packetize” ions therefrom for provision to an ion accelerating stage 354. Buncher 350 may operate in similar fashion to the buncher 340 stage of FIG. 4, and the accelerating stage 354 may operate in similar fashion to the accelerating stage 328 of FIG. 4. The buncher 350 receives the DC ion beam 352 having a uniform energy distribution and groups ions therefrom into bunches 356 of ions within a particular desired energy range, which may then be efficiently accelerated to a desired second energy (e.g., or energy range) by the accelerator 354.

[0028] Another aspect of the invention includes a single electrode buncher having an asymmetrical gap, which further facilitates improved ion transfer efficiency in ion implantation systems. As discussed above, the invention contemplates a single modulating buncher electrode, which may be located between two grounded electrodes (e.g., or grounded focusing devices, such as electrostatic quadrupoles). For instance, the buncher may comprise first and second grounded electrodes, wherein the first grounded electrode is spaced along a beam path from the buncher electrode toward an ion beam entrance end so as to define a first gap therebetween. The second grounded electrode may be spaced along the path from the buncher electrode toward the ion beam exit so as to define a second gap therebetween. According to a further aspect of the invention, one gap may be made larger than the other gap, for example, approximately ten times larger. In addition, one or both of the gaps may be mechanically or otherwise adjustable.

[0029] Referring now to FIG. 6, an exemplary double gap buncher 500 is illustrated along with various components of an ion implantation linear accelerator 528. A DC ion beam (not shown) is provided to the accelerator 528 (erg., from an upstream mass analyzer magnet, not shown), along a beam path 526. The DC beam may comprise, for example, an elongated slit profile that is passed through an entrance aperture 530 having a vertically elongated slit 532 along the path 526. The DC beam is formed into a generally circular profile via two sets of matching quadrupole devices 534 and corresponding grounded electrodes 536, wherein the grounded electrodes 536 each comprise a cylindrical aperture 538 located along the path 526. The linear accelerator 528 further comprises the asymmetrical gap buncher 500 as well as one or more accelerating modules or stages 528 n, where n is an integer, one of which (528 a) is illustrated in FIG. 6. Each of the accelerating modules 528 n further accelerates ions from the beam beyond the energies they achieve from prior modules.

[0030] The buncher 500 comprises a pair of grounded electrodes 546 a and 546 b located before and after a modulating buncher electrode 548 along the path 526, where the modulating buncher electrode 548 may be energized by an appropriate RF energy source and resonator (not shown) in order to modulate ions within the beam along the beam path 526. The grounded electrodes 546 a and 546 b are spaced from the modulating electrode 548 to provide first and second gaps 550 a and 550 b, respectively, therebetween.

[0031] One of the two gaps (e.g., the first gap 550 a) has a length such that the ion transit time through the gap is small compared to the RF period associated therewith. An ion passing through this gap experiences a maximum energy change slightly less than the product of its charge and the peak RF voltage applied to the energized buncher electrode 548. The other gap (e.g., the second gap 550 b) has a substantially greater length, so that the ion transit time through the gap is a large fraction of the RF period. An ion passing through this gap experiences a maximum energy change substantially less than the product of its charge and the peak RF voltage applied to the energized buncher electrode 548. Thus, most of the energy modulation of the ion beam occurs in the shorter of the two gaps.

[0032] In a conventional equal gap buncher, the maximum energy modulation in each gap is equal. In this case, the bunching efficiency depends strongly on the ion transit time from the center of one gap to the center of the other gap. Efficiency is highest when the gap-to-gap transit time is one half of an RF period, as the second gap modulation enhances the first gap modulation. Efficiency is lowest when the gap-to-gap transit time is one, RF period, as the second gap modulation tends to cancel the first gap modulation. The use of unequal length gaps greatly reduces the dependence of buncher efficiency on the gap-to-gap transit time, making efficiency nearly independent of ion species.

[0033] It should be understood that wide variations in gap length may be employed and are contemplated as falling within the scope of the present invention. Conceptually, it is the asymmetry of the gaps that allows one gap to contribute to bunching and the other gap to play a lesser role than conventional, symmetrical gap systems in reducing or undoing the advantageous bunching provided by the first gap mentioned. Further, although in the above example the first gap 550 a is less than the second gap 550 b, it should be understood that the second gap may be made smaller than the first gap, in which case the functions thereof may switch.

[0034] It will be appreciated that one or both of the gaps 550 a, 550 b may be adjusted in order to tune the buncher 500 to ion species. In addition, the power source energizing the modulating electrode 548 may be adjustable (e.g., with respect to frequency, phase, amplitude, etc.) in order to provide appropriate modulation according to one or more system performance parameters. Thus, the exemplary buncher 500 is adaptable for use with accelerators and ion implantation systems optimized for accelerating and implanting specific ion species, as well as those systems wherein the accelerator itself is adjustable.

[0035] The accelerating module or stage 528 a comprises a first grounded electrode 556 located along the path 526 upstream of an energizable electrode 558, as well as a second grounded electrode (not shown) downstream of the energizable electrode 558. The energizable electrode 558 may be energized by an appropriate power source and resonator (not shown) to affect acceleration of ions to a second energy level along the path 526 in a controlled fashion. A set of four focusing quadrupoles 564 may be provided along the path 526 between the buncher stage 500 of the accelerator system 528 and the first accelerator stage 528 a in order to provide radial focusing of the beam as it travels through successive accelerator stages 528 n. The accelerator system 528 may comprise further accelerator stages or modules 528 n (not shown), whereby an accelerated ion beam may be generated at an energy level higher than that of the DC beam provided to the accelerator 528.

[0036] Referring now to FIG. 7, the linear accelerator system 528 is illustrated schematically, wherein a transversely elongated DC ion beam 524 a is provided to the entrance aperture 530 (e.g., having an elongated slit opening). The elongated profile of the beam 524 a is shaped by matching quadrupoles 534 into a generally circular beam 524 b, after which the beam 524 b is provided to the exemplary buncher stage 500 of the accelerator system 528. The buncher stage 500 groups or bunches ions from the beam 524 b into a bunched ion beam 524 c As discussed above, the focusing quadrupoles 564 following the buncher stage 500 maintain this circular profile, and may further provide a drift region to facilitate bunching. The bunched beam 524 c is then provided to the accelerating stages 528 a-528 n of the accelerator system 528, whereby an accelerated ion beam 524 d is obtained, having an energy greater than that of the original DC beam 524 a.

[0037] According to another aspect of the present invention, a buncher is provided, which may operate directly on an ion beam (e.g., beam 524 a) having a transversely elongated longitudinal profile. Referring now to FIGS. 8 and 9, a portion of another exemplary linear accelerator 628 is illustrated having an exemplary slit double gap buncher stage 600 of the accelerator system 628 positioned along a beam path 626 between an entrance aperture 630 having an elongated slit opening 632 therein, and subsequent matching quadrupoles 634 and associated grounded electrodes 636. One or more accelerating stages or modules 628 a-628 n of the accelerator system 628 are located downstream of the quadrupoles 634 along the path 626, one of which (e.g., 628 a) is illustrated in FIG. 8.

[0038] The exemplary slit double gap buncher 600 comprises a single modulating buncher electrode 648, which may be located between the entrance aperture 630 and a grounded electrode 636 associated with the quadrupole devices 634, wherein the electrode 648 includes a transversely elongated slit aperture 604. The buncher electrode 648 is spaced from the entrance aperture 630 so as to define a first gap 650 a therebetween. The grounded electrode 636 may be spaced along the path 626 from the buncher electrode 648 toward the accelerating stage 628 a so as to define a second gap 650 b therebetween. According to a further aspect of the invention, the second gap 650 b may be made larger than the first gap 650 a to obtain the advantages associated with asymmetrical gaps highlighted above, for instance, approximately at least ten times larger than the first gap 650 a. As discussed above, the asymmetry of the gaps may further be selected to be a function of the ion transit time through the gap and the modulation frequency, which dictates the modulation voltage period.

[0039] The slit aperture 604 of the modulating buncher electrode 648 allows the introduction of a ribbon shaped ion beam directly into the buncher 600 without requiring preliminary shaping (e.g., via matching quadrupole devices) into a circular profile. Thus, the buncher stage 600 may be located immediately subsequent the entrance aperture 630 along the beam path 626. This provides for a reduction in the physical length of the accelerator system 628. The elongated-profile ion beam may be thus shaped into a generally circular profile, if desired, subsequent to bunching via the slit double gap buncher 600, using the matching quadrupoles 634 and associated grounded electrodes 636 prior to introduction into the initial accelerator stage 628 a. In this configuration, the quadrupoles 634 may serve both to perform radial shaping of the ion beam along the path 626, as well as to provide a drift region (e.g., or a portion thereof) along which the ions may be advantageously grouped or bunched, as discussed above.

[0040] It will be appreciated that one or both of the gaps 650 a, 650 b may be adjusted in order to tune the buncher stage 600 to specific ion species. In addition, the power source (not shown) energizing the modulating electrode 648 may be adjustable (e.g., with respect to frequency, phase, amplitude, etc.) in order to provide appropriate modulation according to one or more system performance parameters.

[0041] The accelerating module or stage 628 a of the accelerator system 628 comprises a first grounded electrode 656 located along the path 626 upstream of an energizable electrode 658, as well as a second grounded electrode (not shown) downstream of the energizable electrode 658. The energizable electrode 658 may be energized by an appropriate power source and resonator (not shown) to affect acceleration of ions to a second energy level along the path 626 in a controlled fashion. Matching quadrupole devices 634 (e.g., electrostatic quadrupoles) may be provided along the path 626 between the buncher stage 600 and the first accelerating stage 628 a in order to provide radial focusing of the beam as it travels through successive accelerator stages 628 n. Thus, a bunched beam having an elongated profile from the buncher 600 may be shaped into a generally cylindrical beam via the quadrupoles 634 prior to presentation to the first accelerating stage 628 a. The accelerator 628 may comprise further accelerating stages or modules 628 n (not shown), whereby an accelerated ion beam may be generated at an energy level higher than that of the DC beam provided to the accelerator 628.

[0042] Referring also to FIG. 9, a transversely elongated DC ion beam 624 a is provided to the entrance aperture 630 (e.g., having an elongated slit opening). The elongated beam 624 a is then provided to the exemplary slit double gap buncher 600. As discussed above, the elongated aperture 604 of the buncher electrode 648 allows the buncher stage 600 to operate directly on the elongated profile DC ion beam 624 a. The buncher 600 groups or bunches ions from the beam 624 a into a longitudinally bunched ion beam 624 b, which likewise may have an elongated profile. Thereafter, the bunched ion beam 624 b is provided to the matching quadrupoles 634 along the path 626, which shape the elongated beam 624 b into a circular profile ion beam 624 c. As discussed above, the quadrupoles 634 following the buncher stage 600 may also operate to provide a drift region in which the bunching of ions is facilitated. The bunched beam 624 c is then provided to the accelerating stages 628 a-628 n of the accelerator system 628, whereby an accelerated ion beam 624 d is obtained, having an energy greater than that of the original DC beam 624 a.

[0043] Although the invention has been shown and described with respect to a certain aspects and implementations, it will be appreciated that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “including”, “has”, “having”, and variants thereof are used in either the detailed description or the claims, these terms are intended to be inclusive in a manner similar to the term “comprising”. 

What is claimed is:
 1. An ion buncher for grouping ions along a path between an ion beam accelerator entrance and a first accelerating module entrance, the buncher comprising: a buncher electrode located along the path and operatively associated with a buncher energy source and adapted to create a modulating electric field along a portion of the path to provide bunched ions to an accelerating module along the path; and first and second grounded electrodes, wherein the first grounded electrode is spaced along the path from the buncher electrode toward the ion beam accelerator entrance so as to define a first gap therebetween, and wherein the second grounded electrode is spaced along the path from the buncher electrode toward the accelerating module so as to define a second gap therebetween, wherein the first and second gaps are different lengths.
 2. The ion buncher of claim 1, wherein the second gap is larger than the first gap.
 3. The ion buncher of claim 1, wherein the first and second gaps are selected such that one of the first and second gaps comprises a distance sufficiently small such that the modulating electric field therein is generally uniform during a time in which an ion travels therethrough.
 4. The ion buncher of claim 3, wherein the other of the first and second gaps comprises a distance larger than the one gap, such that the modulating electric field therein is generally non-uniform during a time in which the ion travels therethrough.
 5. The ion buncher of claim 2, wherein one of the first and second gaps is adjustable.
 6. The ion buncher of claim 2, wherein the buncher energy source is independently adjustable to vary at least one of the frequency, phase, and field strength associated with the modulating electric field in the portion of the path.
 7. The ion buncher of claim 1, wherein one of the first and second gaps is adjustable.
 8. The ion buncher of claim 1, wherein the buncher energy source is independently adjustable to vary at least one of the frequency, phase, and field strength associated with the modulating electric field in the portion of the path.
 9. A linear accelerator system for accelerating ions traveling along a path from an entrance end to an exit end thereof, the linear accelerator system comprising: at least one accelerating module having at least one energizable electrode operatively associated with an accelerator energy source and adapted to create an accelerating alternating electric field to accelerate the ions to a second energy; and an ion buncher for grouping ions along a path between the entrance end and the at least one accelerating module, the buncher comprising: a buncher electrode located along the path and operatively associated with a buncher energy source and adapted to create a modulating electric field along a portion of the path to provide bunched ions to the at least one accelerating module along the path; and first and second grounded electrodes, wherein the first grounded electrode is spaced along the path from the buncher electrode toward the entrance end so as to define a first gap therebetween, and wherein the second grounded electrode is spaced along the path from the buncher electrode toward the at least one accelerating module so as to define a second gap therebetween, wherein the first and second gaps are different lengths.
 10. The linear accelerator system of claim 9, wherein the first and second gaps are selected such that one of the first and second gaps comprises a distance sufficiently small such that the modulating electric field therein is generally uniform during a time in which an ion travels therethrough.
 11. The linear accelerator system of claim 10, wherein the other of the first and second gaps comprises a distance larger than the one gap, such that the modulating electric field therein is generally non-uniform during a time in which the ion travels therethrough.
 12. An ion implanter comprising: an ion source adapted to direct charged ions having an initial energy along a path; a linear accelerator comprising: at least one accelerating module having at least one accelerating electrode operatively associated with an accelerator energy source and adapted to create an accelerating alternating electric field to accelerate the ions to a second energy; and an ion buncher located between an entrance end of the linear accelerator and the at least one accelerating module, and comprising: a buncher electrode located along the path and operatively associated with a buncher energy source and adapted to create a modulating electric field along a portion of the path to provide bunched ions to the at least one accelerating module along the path; and first and second grounded electrodes, wherein the first grounded electrode is spaced along the path from the buncher electrode toward the entrance end so as to define a first gap therebetween, and wherein the second grounded electrode is spaced along the path from the buncher electrode toward the at least one accelerating module so as to define a second gap therebetween, wherein the first and second gaps are different lengths; an end station adapted to position a workpiece so that charged ions accelerated to the second energy impact the workpiece; and a controller coupled to the energy sources and adapted to control the relative amplitude and phase of the electric fields in the linear accelerator.
 13. The linear accelerator system of claim 12, wherein the second gap is greater than the first gap.
 14. The linear accelerator system of claim 12, wherein the first and second gaps are selected such that one of the first and second gaps comprises a distance sufficiently small such that the modulating electric field therein is generally uniform during a time in which an ion travels therethrough.
 15. The linear accelerator system of claim 14, wherein the other of the first and second gaps comprises a distance larger than the one gap, such that the modulating electric field therein is generally non-uniform during a time in which the ion travels therethrough. 